Water worlds? Astronomers have detected the most Earth-like planet that has the potential to support water yet found by NASA's Kepler Space Telescope.

The planet, known as Kepler 62f, is one of five new planets discovered orbiting a star in the constellation of Lyra.

Kepler 62f is likely to be a terrestrial planet about 40 per cent bigger than Earth, orbiting in the star's habitable zone -- the area that allows liquid water to exist, the astronomers report in the journal Science.

"We're very excited because this has a good chance of being a rocky planet and potentially having the right surface temperatures to support liquid water," says Associate Professor Eric Agol, of the University of Washington, one of the paper's authors.

The planet receives about half as much heat and light as Earth and orbits its host star in 267.3 Earth days.

It's one of two 'super-Earths' discovered in the planet system's habitable zone.

A super-Earth is a planet greater in mass than our own, but still smaller than ice giants such as Neptune.

Kepler 62's other super-Earth, nearby 62e, is bigger at 1.61 times Earth's size, orbits the star in just 122.4 days, and gets about 20 per cent more light and heat than Earth.

The two newly discovered worlds are the smallest exoplanets yet found in a habitable zone.

While the sizes of Kepler 62e and 62f are known, their mass and densities are not.

"All the other planets which we found to date for which we can measure the mass have high densities there similar in density to the Earth," says Agol.

"Based on its size, our best guess is that Kepler 62f is rocky and has some atmosphere, but not a thick gaseous envelope, like Neptune."

NASA's Kepler spacecraft, which was launched in 2009 searches for Earth-like planets orbiting 170,000 Sun like stars by staring continuously at a single section of the sky looking for subtle changes in light caused by a planet passing in front of its star.

According to Agol, Kepler 62f was a late discovery.

"I've been working on a new technique for searching for transiting planets in the Kepler data, and figured that I'll try this on the new planet system that they're about to submit for publication," says Agol.

Kepler needs three transits to confirm a planetary candidate, but only had two for Kepler 62f so the planet was initially dismissed as a glitch in the data.

"But my algorithm went along and ignored any such subtleties in the data and managed to identify three transits...to produce a planet candidate that hadn't been identified by the Kepler software," says Agol.

The super-Earth's siblings Kepler 62 b, c and d are 1.31, 0.54 and 1.95 times the size of the Earth, respectively, but orbit the star too close to be in the habitable zone.

The host star, Kepler-62 is an orange dwarf, slightly smaller and cooler than the Sun and more than one and a half times older.

Endless oceans

A separate study accepted for publication in The Astrophysical Journal suggests both Kepler 62e and 62f are water worlds covered by global oceans.

"There may be life there, but...life on these worlds would be under water."

"Kepler-62e probably has a very cloudy sky and is warm and humid all the way to the polar regions. Kepler-62f would be cooler, but still potentially life-friendly," says Harvard astronomer and co-author Professor Dimitar Sasselov.

Next generation explorers

The findings are a continuation of the many exciting results coming out from NASA's Kepler satellite, says astronomer Dr Chris Tinney from the University of New South Wales.

"They've been finding planets that are increasingly more and more like the ones that are in our own solar system," says Tinney.

But, he says, there is a limit to what Kepler can detect.

"The Kepler experiment is great, but the stars that it sees are fairly faint."

Tinney says a new generation planet hunting spacecraft called the Transiting Exoplanet Survey Satellite or TESS just approved by NASA will build on Kepler's achievements.

"Kepler...stares at one field continuously, where as the TESS experiment will look at many more stars...much brighter ones."

This will allow detailed follow up work to be carried out on newly discovered systems, according to Tinney.

"This will be possible with TESS, where it's very difficult to impossible with Kepler," he says.